The separation of various substances through selective adsorption is an important process for producing pure substances. However, this generally is a batch process, but with the development of simulated moving bed (SMB) technology, the adsorption separation process can be operated on a continuous basis. For simulated moving bed technology, the process uses a multiport rotary valve to redirect flow lines in the process. The simulation of a moving adsorbent bed is described in U.S. Pat. No. 2,985,589 (Broughton et al.). In accomplishing this simulation, it is necessary to connect a feed stream to a series of beds in sequence, first to bed no. 1, then to bed no. 2, and so forth for numerous beds, the number of beds often being between 12 and 24. These beds may be considered to be portions of a single large bed whose movement is simulated. Each time the feed stream destination is changed, it is also necessary to change the destinations (or origins) of at least three other streams, which may be streams entering the beds, such as the feed stream, or leaving the beds. The moving bed simulation may be simply described as dividing the bed into series of fixed beds and moving the points of introducing and withdrawing liquid streams past the series of fixed beds instead of moving the beds past the introduction and withdrawal points. A rotary valve used in the Broughton process may be described as accomplishing the simultaneous interconnection of two separate groups of conduits.
There are many different process requirements in moving bed simulation processes, resulting in different flow schemes and thus variations in rotary valve arrangement. For example, in addition to the four basic streams described in Broughton (U.S. Pat. No. 2,985,589), it may be desirable to utilize one or more streams to purge, or flush, a pipeline or pipelines. A flush stream is used to prevent undesirable mixing of components. The flush substance is chosen to be one which is not undesirable for mixing with either main stream, that being purged or that which enters the pipeline after flushing is completed. U.S. Pat. No. 3,201,491 (Stine et al.) may be consulted for information on flushing lines as applied to the process of Broughton (U.S. Pat. No. 2,985,589). It may be desirable to pass fluid through a bed or beds in the reverse direction from normal flow. This is commonly known as backflushing, a subject treated in U.S. Pat. No. 4,319,929 (Fickel). Other applications for various arrangements of multiport rotary disc valves may be seen in U.S. Pat. No. 4,313,015 (Broughton); U.S. Pat. No. 4,157,267 (Odawara et al.); U.S. Pat. No. 4,182,633 (Ishikawa et al.); and U.S. Pat. No. 4,409,033 (LeRoy).
While the multiport rotary disc valve of Carson (U.S. Pat. No. 3,040,777) provided a satisfactory valve design for the simultaneous interconnection of two independent groups of conduits such that each conduit of the first group could be brought into individual communication with every conduit of the second group, it is not suitable when three groups of conduits must be simultaneously interconnected in the same manner. Upon reference to Broughton (U.S. Pat. No. 2,985,589), it can be seen that there are only two groups of conduits which need to be interconnected when the arrangement of the drawing of that patent is utilized. One group consists of the conduits which provide the flows entering and leaving the simulated moving bed adsorbent system, that is, the flows which are switched among the beds, such as the feed stream. A second group consists of the conduits associated with the individual beds, that is, which supply and remove fluid from the beds, one conduit being connected between each two beds. It is to be noted that each conduit of the second group serves that dual function of supply and removal, so that it is unnecessary to provide conduits for supplying fluid separate from those for removing fluid.
When it is necessary to simultaneously interconnect conduits of three different groups of conduits in accordance with a previously determined cycle, the apparatus of the present invention may be used. An example of process involving three conduit groups may be found in U.S. Pat. No. 4,402,832 (Gerhold), which is described below. As mentioned above, it is highly desirable to use a single device to do so, thereby avoiding the obvious problems associated with numerous separate valves which must be simultaneously actuated.
One of the issues associated with simulated moving bed technology and rotary valves is the need for cross-over lines to make the appropriate connections when the rotary valve shifts the source of the feed inlets and the drawoff outlets relative to the bed. The cross-over lines often need to be long and create back mixing problems.
An alternative to simulated moving beds is a true moving bed wherein an adsorbent bed is moved to regions of different operating conditions to change from adsorption to desorption. One such example is an adsorbent wheel as shown in U.S. Pat. No. 5,685,897. The adsorbent wheel has two parts where air flowing over one part in a first region deposits moisture in the adsorbent and as the adsorbent is moved to a second region the moisture is given up to another air stream. Alternate designs exist for what are essentially adsorbent wheels wherein individual adsorbent beds are moved through different operating regions to purify a gas as shown in U.S. Pat. No. 5,080,700. Another form of a true moving bed involves a continuous rotating annular chromatograph, as shown in U.S. Pat. No. 5,130,001. An annular adsorbent bed moves under a feedstream inlet and at least one eluent inlet. As the material travels through the adsorbent bed there is separation of a mixture, and when the material leaves the bottom of the adsorbent bed, it is collected in a series of collection outlets.
There is substantial room for improvement in design that can simplify the adsorption separation device structure and also reduce back mixing.